[PATCH v3] drm/dp: clamp PWM bit count to advertised MIN and MAX capabilities

[Christopher Obbard]

According to the eDP specification (VESA Embedded DisplayPort Standard
v1.4b, Section 3.3.10.2), if the value of DP_EDP_PWMGEN_BIT_COUNT is
less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, the sink is required to use
the MIN value as the effective PWM bit count.

This commit updates the logic to clamp the reported
DP_EDP_PWMGEN_BIT_COUNT to the range defined by
DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN and _CAP_MAX. This ensures correct
handling of eDP panels that report a zero PWM bit count but still
provide valid non-zero MIN and MAX capability values. Without this
clamping, brightness values may be interpreted incorrectly, leading
to a dim or non-functional backlight.

For example, the Samsung ATNA40YK20 OLED panel used in the Lenovo
ThinkPad T14s Gen6 (Snapdragon) reports a PWM bit count of 0, but
supports AUX backlight control and declares a valid 11-bit range.
Clamping ensures brightness scaling works as intended on such panels.

Co-developed-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
Signed-off-by: Rui Miguel Silva <rui.silva@xxxxxxxxxx>
Signed-off-by: Christopher Obbard <christopher.obbard@xxxxxxxxxx>
---
Changes in v3:
- Properly rebase patch on top of latest version of drm-misc-next.
- Make patch more generic by clamping PWM bit count to advertised MIN
  and MAX capabilities (suggested by Dmitry).
- Link to v2: https://lore.kernel.org/r/20250327-wip-obbardc-qcom-t14s-oled-panel-brightness-v2-1-16dc3ee00276@xxxxxxxxxx

Changes in v2:
- Split backlight brightness patch from T14s OLED enablement series.
- Use PWMGEN_CAP_MIN rather than MAX (Dmitry).
- Rework commit message to reference eDP spec.
- Rebase on drm-misc-next.
- Link to v1: https://lore.kernel.org/all/20250325-wip-obbardc-qcom-t14s-oled-panel-v2-4-e9bc7c9d30cc@xxxxxxxxxx/
---
 drivers/gpu/drm/display/drm_dp_helper.c | 48 ++++++++++++++++++++-------------
 1 file changed, 30 insertions(+), 18 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index e2439c8a7fefe116b04aaa689b557e2387b05540..fcc26cb96a51066a503433b2dc660126155d179c 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -28,6 +28,7 @@
 #include <linux/init.h>
 #include <linux/iopoll.h>
 #include <linux/kernel.h>
+#include <linux/minmax.h>
 #include <linux/module.h>
 #include <linux/sched.h>
 #include <linux/seq_file.h>
@@ -4033,8 +4034,33 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
 			    aux->name, ret);
 		return -ENODEV;
 	}
-
 	pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
+	if (ret != 1) {
+		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
+			    aux->name, ret);
+		return -ENODEV;
+	}
+	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
+	if (ret != 1) {
+		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
+			    aux->name, ret);
+		return -ENODEV;
+	}
+	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	/*
+	 * Per VESA eDP Spec v1.4b, section 3.3.10.2:
+	 * If DP_EDP_PWMGEN_BIT_COUNT is less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN,
+	 * the sink must use the MIN value as the effective PWM bit count.
+	 * Clamp the reported value to the [MIN, MAX] capability range to ensure
+	 * correct brightness scaling on compliant eDP panels.
+	 */
+	pn = clamp(pn, pn_min, pn_max);
+
 	bl->max = (1 << pn) - 1;
 	if (!driver_pwm_freq_hz)
 		return 0;
@@ -4054,29 +4080,15 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
 	 */
 	fxp = DIV_ROUND_CLOSEST(1000 * DP_EDP_BACKLIGHT_FREQ_BASE_KHZ, driver_pwm_freq_hz);
 
-	/* Use highest possible value of Pn for more granularity of brightness adjustment while
+	/*
+	 * Ensure frequency is within 25% of desired value.
+	 * Use highest possible value of Pn for more granularity of brightness adjustment while
 	 * satisfying the conditions below.
 	 * - Pn is in the range of Pn_min and Pn_max
 	 * - F is in the range of 1 and 255
 	 * - FxP is within 25% of desired value.
 	 *   Note: 25% is arbitrary value and may need some tweak.
 	 */
-	ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
-	if (ret < 0) {
-		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
-			    aux->name, ret);
-		return 0;
-	}
-	ret = drm_dp_dpcd_read_byte(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
-	if (ret < 0) {
-		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
-			    aux->name, ret);
-		return 0;
-	}
-	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
-	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
-
-	/* Ensure frequency is within 25% of desired value */
 	fxp_min = DIV_ROUND_CLOSEST(fxp * 3, 4);
 	fxp_max = DIV_ROUND_CLOSEST(fxp * 5, 4);
 	if (fxp_min < (1 << pn_min) || (255 << pn_max) < fxp_max) {

---
base-commit: 4c4d9b7b6c6e676eca22585139aba5f03de74b90
change-id: 20250327-wip-obbardc-qcom-t14s-oled-panel-brightness-4020865b6580

Best regards,
-- 
Christopher Obbard <christopher.obbard@xxxxxxxxxx>